This invention relates to devices for testing insulated electrical wire or cable, usually either during or immediately after manufacture, to search for faults in the insulation and deals more particularly with such devices wherein an electrical potential is applied to the outer surface of the insulation jacket by a suitable liquid electrode and faults such as pinholes or voids are detected by sensing the current supplied to the electrode.
Insulated electrical conductors, such as wire or cable, are usually subjected to testing prior to use to check for any unseen imperfections which may be present in the insulation. One common method of testing is to pass the insulated conductor, which is suitably grounded, through an electrode of some kind such as a beaded chain curtain which applies a high test voltage to the outer surface of the insulation. Any fault in the insulation will result in an arc or spark between the conductor of the wire and the electrode at the point in the insulation where the fault occurs causing a current to flow in the conductor to ground. Movement of the wire through the curtain may cause the curtain to move out of contact with the insulation surface thereby creating an air gap between the electrode and the wire. However, since relatively high test voltages are used in spark testing devices, the air gap between the electrode and the fault is arced causing a current to flow in the conductor to ground. The voltages of such spark testing devices often exceed the dielectric strength of smaller diameter, more fragile wire such as telecommunication and computer wire which may blow holes in the insulation during testing. Accordingly, it is desirable to have a wire testing apparatus using lower test voltages to test lower dielectric strength insulated wire.
When low test voltages are used with spark type testing devices of the beaded curtain type, unreliable test results often occur since the electrode may not maintain continuous surface contact with the insulation surface as the wire moves through the beaded curtain. The low test voltages are generally insufficient to arc the air gap to cause current to flow in the conductor to ground to detect the fault. Additionally, changing electrode characteristics caused by dirt accumulation, wear and changes in ambient humidity affect the reliability of electrode contact with the insulation surface.
One approach to solving the aforedescribed problem is to use a confined liquid bath as the electrode. One such liquid bath testing device requires immersing the wire in the bath and applying a high alternating test voltage between the wire and the liquid and measuring the current flowing in the conductor to determine the overall condition of the insulation; however, this device fails to detect individual faults. Another liquid bath testing device requires altering or bending the path along which the wire travels to accommodate entry to and exit from the liquid bath which can reduce the speed of the wire passing through the bath. Still other liquid bath type testing devices having specific ionic conductivity levels and temperatures are more complex and more difficult to maintain.
Accordingly, it is the general object of this invention to provide an improved low voltage wire insulation testing apparatus having a liquid electrode.
Other objects and advantages of the invention will be apparent from the following written description and from the drawing forming a part hereof.